This invention relates in general to video tape reproduction apparatus such as helical scan video cassette recorders. More particularly, this invention relates to a video tape reproduction apparatus which is operable in a high quality mode in which a field of video information is recorded on two adjacent helically scanned tracks on tape. The second half of the field is delayed, when it is recorded on the second track, in order to eliminate skew-induced gaps in the signal as playback is switched from the first to the second track.
The standard broadcast color video signal has a greater bandwidth than is generally utilized in conventional television receivers and video cassette recorders. Thus, the composite NTSC color television signal has a luminance signal with a bandwidth of approximately 4.2 MHz and chrominance signals with bandwidths as great as 1.3 MHz. When the NTSC video signal is utilized in a VCR, the bandwidth of the luminance and chrominance signals are reduced in order to achieve acceptably long playing time and low video noise in conjunction with a reasonable tape cassette size. This reduction in bandwidth, however, results in a reduction in image resolution and quality.
It has been proposed in commonly-assigned, copending U.S. patent application Ser. No. 725,873 entitled VIDEO REPRODUCTION APPARATUS, filed Apr. 22, 1985, by Carl Schauffele, to provide a video reproduction apparatus which may be operated in either a conventional mode or a high quality mode. In the conventional mode, a color under format color video signal of reduced bandwidth is helically recorded on magnetic tape such that each field of the video signal is recorded on a single track on the magnetic tape. In the high quality mode, in a preferred embodiment of the invention, a time division multiplex (TDM) format color video signal of increased bandwidth is recorded on tape which is moved twice as fast as the speed of the tape in the conventional format recording, and with a magnetic transducer which is rotated at twice the rotational speed as in the conventional format mode. Thus, a field of video information in the TDM format is recorded on two adjacent tracks, the upper half of a video field being recorded on one track and the lower half of the field being recorded on the other track.
The video reproduction apparatus disclosed in the latter patent application is highly advantageous in giving a VCR operator the choice of recording either in a high quality mode, or in a conventional mode. Difficulties may arise, however, during playback of the TDM format video signal due to skew errors caused when switching between signals successively reproduced from the two tracks of a field. Skew errors result from differences in tension of the magnetic tape between the time of recording a video signal on the tape and the time the video signal is played back causing bunching up or stretching out of horizontal lines of signals. At the head switching point, a skew error of several microseconds can affect horizontal synchronization in the reproduced scene causing discontinuities and distortions in the televised image. The most serious of those discontinuities occurs if the skew error is such that the video signal on the second track of a field, ready by the second head, slightly precedes that on the first track. In this case, when playback is switched from track one to track two, an interval of the picture signal is uncorrectably lost. Since the switching occurs in the middle of the viewing area, image degradation is particularly annoying. Skew error is especially critical in the disclosed time division multiplex format, which uses a compressed horizontal sync signal which is substantially shorter in duration than the normal horizontal sync signal of a composite NTSC color video signal. Thus a variation in tape tension will more likely result in a failure to pick up the compressed horizontal sync pulse when switching in mid-field.